1. 1 End-to-End Protocols UDP TCP –Connection Establishment/Termination –Sliding Window Revisited –Flow Control –Congestion Control –Adaptive Timeout

2. 2 End-to-End Protocols Underlying best-effort network –drop messages –re-orders messages –delivers duplicate copies of a given message –limits messages to some finite size –delivers messages after an arbitrarily long delay Common end-to-end services –guarantee message delivery –deliver messages in the same order they are sent –deliver at most one copy of each message –support arbitrarily large messages –support synchronization –allow the receiver to flow control the sender –support multiple application processes on each host

3. 3 Simple Demultiplexor (UDP) Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports –servers have well-known ports –see /etc/services on Unix Header format Optional checksum –psuedo header + UDP header + data SrcPortDstPort ChecksumLength Data 01631

4. 4 TCP Overview Connection-oriented Byte-stream –app writes bytes –TCP sends segments –app reads bytes Full duplex Application process Write bytes TCP Send buffer Segment Transmit segments Application process Read bytes TCP Receive buffer … … … Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning network Adaptive Timeout

5. 5 Transport Versus Data Link Potentially connects many different hosts –need explicit connection establishment and termination Potentially different RTT –need adaptive timeout mechanism Potentially long delay in network –need to be prepared for arrival of very old packets Potentially different capacity at destination –need to accommodate different node capacity Potentially different network capacity –need to be prepared for network congestion

6. 6 Segment Format

7. 7 Segment Format (cont) Each connection identified with 4-tuple: –(SrcPort, SrcIPAddr, DsrPort, DstIPAddr) Sliding window + flow control –acknowledgment, SequenceNum, AdvertisedWinow Flags –SYN, FIN, RESET, PUSH, URG, ACK Checksum –pseudo header + TCP header + data Sender Data(SequenceNum) Acknowledgment + AdvertisedWindow Receiver

8. 8 Connection Establishment and Termination Active participant (client) Passive participant (server) SYN, SequenceNum = x SYN + ACK, SequenceNum = y, ACK, Acknowledgment = y + 1 Acknowledgment = x + 1

9. 9 State Transition Diagram

10. 10 TCP Flow Control and Congestion Control (a) A fast network feeding a low capacity receiver. (b) A slow network feeding a high-capacity receiver.

11. 11 Flow Control Example

12. 12 Sliding Window Revisited Sending side –LastByteAcked < = LastByteSent –LastByteSent < = LastByteWritten –buffer bytes between LastByteAcked and LastByteWritten Sending application LastByteWritten TCP LastByteSentLastByteAcked Receiving application LastByteRead TCP LastByteRcvdNextByteExpected Receiving side –LastByteRead < NextByteExpected –NextByteExpected < = LastByteRcvd +1 –buffer bytes between NextByteRead and LastByteRcvd

13. 13 Flow Control Send buffer size: MaxSendBuffer Receive buffer size: MaxRcvBuffer Receiving side –LastByteRcvd - LastByteRead < = MaxRcvBuffer –AdvertisedWindow = MaxRcvBuffer - ( NextByteExpected - NextByteRead ) Sending side –LastByteSent - LastByteAcked < = AdvertisedWindow –EffectiveWindow = AdvertisedWindow - ( LastByteSent - LastByteAcked ) –LastByteWritten - LastByteAcked < = MaxSendBuffer –block sender if ( LastByteWritten - LastByteAcked ) + y > MaxSenderBuffer Always send ACK in response to arriving data segment Persist when AdvertisedWindow = 0

14. 14 Protection Against Wrap Around 32-bit SequenceNum BandwidthTime Until Wrap Around T1 (1.5 Mbps)6.4 hours Ethernet (10 Mbps)57 minutes T3 (45 Mbps)13 minutes FDDI (100 Mbps)6 minutes STS-3 (155 Mbps)4 minutes STS-12 (622 Mbps)55 seconds STS-24 (1.2 Gbps)28 seconds

15. 15 Keeping the Pipe Full 16-bit AdvertisedWindow BandwidthDelay x Bandwidth Product T1 (1.5 Mbps)18KB Ethernet (10 Mbps)122KB T3 (45 Mbps)549KB FDDI (100 Mbps)1.2MB STS-3 (155 Mbps)1.8MB STS-12 (622 Mbps)7.4MB STS-24 (1.2 Gbps)14.8MB

16. 16 TCP Extensions Implemented as header options Store timestamp in outgoing segments Extend sequence space with 32-bit timestamp (PAWS) Shift (scale) advertised window

17. 17 TCP Congestion Control – Slow Start An example of the Internet congestion algorithm.

18. 18 TCP Timer Management (a) Probability density of ACK arrival times in the data link layer. (b) Probability density of ACK arrival times for TCP.

19. 19 Adaptive Retransmission (Original Algorithm) Measure SampleRTT for each segment/ ACK pair Compute weighted average of RTT –EstRTT =  x EstRTT + (1 -  ) x SampleRTT  between 0.8 and 0.9 Set timeout based on EstRTT –TimeOut = 2 x EstRTT

20. 20 Karn/Partridge Algorithm Do not sample RTT when retransmitting Double timeout after each retransmission SenderReceiver Original transmission ACK SampleR TT Retransmission SenderReceiver Original transmission ACK SampleR TT Retransmission

21. 21 Jacobson/ Karels Algorithm New Calculations for average RTT Difference = SampleRTT - EstRTT EstRTT = EstRTT + (  x Difference) Deviation = Deviation +  (|Difference| - Deviation) –where  is a factor between 0 and 1 Consider variance when setting timeout value TimeOut =  x EstRTT +  x Deviation –where  = 1 and  = 4